Generally, PIN-diode infrared detectors are detector matrices, hybridized with a multiplexed silicon read circuit, which require back-lighting through the lower contact layer.
The lower contact layer (generally an n-type doped semiconductor layer) is a layer said to be a window to the SWIR radiation to be detected but that absorbs visible light, thus attenuating part of the incident flux which could otherwise be absorbed in the working layer of the diode (undoped region).
In general, the various materials used are direct-bandgap semiconductors, with absorption coefficients much higher than 104 cm−1 in this part of the spectrum.
Typically, for an InP-based PIN diode, at an operating wavelength of λ=0.6 μm, the value of the absorption coefficient is 6.7×104 cm−1. This considerably limits the radiation incident on the active region. This is because the thickness of this layer must be enough to ensure a uniform contact (typically this thickness must be greater than at least about 300 nm), thereby already causing an 86% attenuation.
FIG. 1a shows a diagram of a very generic PIN diode and FIG. 1b illustrates a prior-art PIN diode made of GaInAs/InP.
Generally, an active region 13 made of undoped semiconductor is inserted between an n-type doped semiconductor layer 12 and a p-type doped semiconductor layer 14, the array being integrated between a lower electrode 11 and an upper electrode 15.
An exemplary structure is illustrated in FIG. 1b. A backside electrode 21 bears a substrate 221 made of n+-type doped InP covered with a layer 222 of n+-type doped InP. The active detection layer 23 is made of InGaAs. The active layer is covered by a layer 24 made of p+-type doped (Zn) InGaAs. A Ti/Au contact 25 is produced on this p+-type doped layer. Exemplary dimensions are shown in this FIG. 1b. 
One solution currently employed to produce this type of infrared detector consists in, on the one hand, removing the substrate, and on the other hand, above all, limiting the thickness of a part of the contact layer.
Because of the electrical function of this contact layer it is not possible to thin it sufficiently, because a uniform contact must be ensured. For a “reasonable” thickness of about 200 nm, the attenuation is already 74% in a material such as InP at 0.6 μm. In practice, the quantum efficiency of an optimized GaInAs/InP photodiode is 85% in the SWIR and only 30% at 0.6 μm and it approaches 0 at 450 nm.
In this respect, FIG. 2 illustrates the spectral response of an exemplary Goodrich detector. The curve 2a shows the response of a detector extending into the visible range, the curve 2b shows the response of the standard detector in the SWIR range.